1. Field of the Invention
The present application is directed to a method and system for continuously monitoring and controlling a process stream for a dechlorination residual and, in particular, to a method and system for continuously monitoring and controlling a process stream for a dechlorination residual using chloramine-T.
2. Description of the Related Art
Chlorine is commonly used to disinfect sewage treatment plant process streams and for biofouling control in cooling systems. Because chlorine is such a highly effective disinfecting/oxidizing agent, any chlorine unused in the process is just as effective in destroying aquatic life.
In order to eliminate or substantially remove the residual chlorine from a process stream, a dechlorination agent or device is often used. Typically, sulfur dioxide (SO2) is added to the process stream to react quantitatively with the chlorine residual. If SO2 is added in excess of the amount of chlorine residual in the process stream, the chlorine will be completely eliminated.
The amount of unreacted dissolved SO2 (i.e., dechlorination residual) remaining in the process stream is preferably maintained at low, but positive, concentration to ensure chlorine residual removal and minimize oxygen consumed. This sort of treatment not only protects aquatic life, but results in more efficient use of the Cl2 and SO2 reagents added to the process stream. However, there are no completely acceptable methods or systems available for directly monitoring and controlling the amount of dechlorination residual in a continuous process stream to which a chlorine disinfectant/oxidant has been previously added.
One example of a method for determining the concentration of a chemical constituent in a fluid is disclosed in U.S. Pat. No. 2,560,317. The method disclosed is particularly well-suited for detecting the concentration of residual chlorine in a process stream. The method involves the removal of chlorine from a process stream sample stream by passing the sample through an activated carbon bed and thereafter adding chlorine at a variable and determinable rate to compare the relative concentrations of chlorine in that stream to the relative concentration of chlorine in an untreated process sample stream. When the comparison is brought to a zero difference, by adjusting the rate of chlorine addition, the determinable rate at which chlorine is added to the chlorine free sample stream is an exact measure of the amount of chlorine in the process stream. Iodine is identified as a suitable substitute for chlorine when it is added to the chlorine free stream. Although this method allows the amount of residual chlorine to be measured, it is completely silent as to measuring a dechlorination residual in the process stream.
Other examples of methods for measuring and controlling on-line zero chlorine residual are disclosed in Finger, et al., xe2x80x9cDevelopment Of An On-Line Zero Chlorine Residual Measurement And Control System.xe2x80x9d J. Water Pollution Control Fed., Vol. 57, No. 11, 1068 (1985). Finger et al. recognize that there are no techniques available to directly monitor or control zero chlorine residuals nor are there any continuous analytical techniques available to monitor dechlorination (i.e., dissolved SO2) residuals.
Finger et al. identify a feed-forward system that incorporates an effluent flow signal and a chlorine residual signal which are measured immediately before the dechlorination point. Finger et al. also identify a feedback residual control system that biases the dechlorination sample with large volumes of gaseous chlorine or liquid hypochlorite reagents before the dechlorination sample is analyzed. However, Finger et al. argue that both of these systems are deficient because they depend on precise sample and biasing reagent flows. Sample biasing with chlorine or hypochlorite is also subject to chemical reaction interferences with other contaminants (e.g., ammonia) which may be present in the process stream.
In an attempt to overcome these problems, Finger et al. provide a complex feedback approach wherein a dechlorination effluent is biased with the chlorinated effluent to form a sample that can be measured with conventional chlorine analyzers. The effluent flows are held at a 1:1 ratio with constant-head tanks and the chlorine residual of the dechlorination effluent is electronically calculated from the measured residuals in the chlorine and dechlorinated effluent based on the equation:
Cpostxe2x88x922Cmeasxe2x88x92Cpre
wherein:
Cpost=the chlorine residual of the dechlorinated effluent;
Cpre=the chlorine residual of the effluent prior to dechlorination; and
Cmeas=the chlorine residual of the 1:1 mixture of pre- and post-dechlorination effluents.
The method requires two analyzers to be used to determine the amount of chlorine residual in the dechlorinated effluent. Excessive instrumentation is necessary because the variability of the residual measuring technique must be minimized to ensure control within regulatory limits. The dechlorinated residual is calculated and controlled from the summation output of the two residual analyzers.
All of the systems discussed above are expensive to install and have substantial operational costs associated with maintenance, sample pumping, and reagent costs. These systems are often inherently unstable, producing measurement errors that typically exceed safe regulatory residual limits by several orders of magnitude. This is especially undesirable since dechlorination control continues to be the subject of more stringent environmental regulations.
The invention comprises methods and systems for monitoring and controlling the amount of a disinfectant removing agent, for example, sulfur dioxide, in a continuous process stream. The disinfectant removing agent is added in excess to the process stream to reduce or eliminate a disinfectant residual which has previously been added to the process stream.
The method of the present invention comprises continuously drawing off a sample of the process stream and continuously adding an iodine analyzing agent to the sample in an amount sufficient to completely react with the residual disinfectant removing agent and leave an unreacted amount of analyzing agent or, in the case of incomplete disinfectant removal, to add to the disinfectant residual. The iodine is provided by a reaction between two reagents, an iodide and chloramine-T (C7H7ClNO2Sxe2x88x92Na+). The two reagents are introduced into the sample stream which is maintained at a pH from about 3.5 to about 4.5 through the introduction of a third reagent, an acid. The three reagents may be introduced into the sample stream separately or together and the time required to react, i.e., to produce the iodine, is minimal. Thereafter, the sample is continuously analyzed to determine the amount of unreacted iodine, or iodine plus disinfectant, remaining in the sample. Based on the amount of iodine added and the amount of unreacted iodine remaining in the sample after reaction, the amount of residual disinfectant removing agent or disinfectant residual is determined. Using the determined amount of residual disinfectant or disinfectant removing agent, the amount of disinfectant removing agent that is added to the process stream is controlled.